A sea lion (Zalophus californianus) and a porpoise (Tursiops truncatus) were trained to report the presence of a pure tone under water in yes–no psychophysical procedures. Signal probability was held constant at 0.50. For the sea lion signal strength and payoff matrix were varied concurrently while only payoff matrix was varied for the porpoise. Payoff matrix was manipulated by changing the amount of reinforcement (number of fish) consequent on the two different classes of correct responses—hits and correct rejections. In terms of the ratio of hits to correct rejections the matrix was varied over three values—1:1, 4:1, and 1:4. In the sea lion signal detection improved as signal intensity increased and was independent of the sea lion’s response bias. In the porpoise changes in response bias occurrred as a function of changes in the payoff matrix. Both the sea lion and the porpoise repeatedly demonstrated rapid acquisition of a stable response bias. These experiments demonstrate that varying the payoff matrix may be an effective way to control response bias in experiments dealing with the detection of underwater signals by marine mammals.
Subject Classification: 80.50, 80.60.
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